A conventional image reading apparatus has an auto document feeder (ADF) function. The image reading apparatus conveys document sheets stacked on a tray using a feed roller, separates the document sheets one by one using a separation roller, and conveys the separated sheet to an image sensor using a conveyance roller. A sensor is disposed between the conveyance roller and the image sensor for detecting presence of a sheet. The feed roller and the conveyance roller are driven by a single motor, and a clutch is disposed between the motor and the feed roller so as to transmit/intercept a driving force from the motor.
A reading speed of the image reading apparatus varies depending on a desired image reading resolution. The image reading apparatus has three operation modes depending on the image reading resolution. A first operation mode corresponds to a high resolution reading. In the first operation mode, after the image reading apparatus ends an image reading of a preceding sheet, the feed roller starts feeding a subsequent sheet at a speed higher than a reading speed. Thereafter, when the sensor detects a leading edge of the subsequent sheet, the feed roller is stopped and the conveyance roller conveys the subsequent sheet to the image sensor at a low speed, which is the reading speed in this case. A second operation mode corresponds to a middle resolution reading. In the second operation mode, after the sensor detects a trailing edge of a preceding sheet, the feed roller starts feeding a subsequent sheet at a feed speed substantially equal to the reading speed. After the reading of an image of the preceding sheet ends, the feed roller increases the feed speed of the subsequent sheet to a higher speed. Thereafter, when the sensor detects the leading edge of the subsequent sheet, the conveyance roller conveys the subsequent sheet at a middle speed, which is the reading speed in this case, to the image sensor. A third operation mode corresponds to a low resolution reading. In the third mode, until a trailing edge of a preceding sheet passes through the position of the sensor, the feed roller does not feed a subsequent sheet. When the sensor detects the trailing edge of the preceding sheet, the feed roller starts feeding the subsequent sheet at the feed speed substantially equal to the reading speed. Thereafter, the feed speed of the subsequent speed by the feed roller is not changed. The conveyance roller conveys the subsequent sheet to the image sensor at a high speed which is the reading speed in this case when the leading edge of the subsequent sheet is detected by the sensor.